Two-way radio voice-communication systems have become extremely popular in recent years, and so much so that the usual prior art voice-communication systems are no longer adequate. Digital and tone signalling equipment are presently in widespread use to provide selective calling in present-day radio voice-communication systems, especially in circumstances where the radio system is interconnected to a telephone exchange. The "Touch-Tone" system developed by American Telephone and Telegraph Company for its telephone switching networks has proven to be adaptable to such radio voice-communication systems.
The usual Touch-Tone encoder is designed so that when a particular pushbutton on the keyboard is depressed, a pair of signalling tones is produced, with each tone being carefully selected to avoid the principal harmonics normally created in speech and music. Touch-Tone signalling has been adapted to radio communication, and has been used satisfactorily in prior art two-way radio voice-communcation systems.
The usual prior art Touch-Tone encoder unit for mobile radio transmitters includes a keyboard and an encoder generator circuit connected to the keyboard. The encoder generator circuit generates the two distinct tones whenever a particular pushbutton on the keyboard is depressed, and these tones serve to identify the particular pushbutton. The dual tones are transmitted by the mobile transmitter to the receivers in the system in which they are decoded. In this way, a call may be placed to a selected receiver by depressing a number of pushbuttons on the keyboard in a particular sequence, corresponding to the selected code for the particular receiver.
The keyboard-encoder unit of the present invention constitutes a useful commercial product which can be readily manufactured for sale at a relatively low price, and which can be easily mounted on a mobile radio transmitter to fulfill its intended function.